Hydrangea Haven
In the Garden with Andrew
Andrew Bunting. Photo courtesy of the Pennsylvania Horticultural Society.
The hydrangea may be the most planted shrub in North America, if not the world. The genus comprises dozens of species and over 1,000 cultivars, selections, and hybrids. One hydrangea or another will grow in every state. Some species are native, while others have been hybridized for decades.
The most well-known of the hydrangeas are the mopheads, or hortensias, which turn pink or blue depending on the acidity of your soil. In the acidic soils common in Swarthmore, you will mostly have blue hydrangeas. But in areas of the Midwest where the soils trend towards alkaline, the hydrangeas will be pink. Years ago, I planted a hydrangea at the Scott Arboretum called ‘Forever Pink’. It was pink the first summer and then “forever blue” thereafter.
Some of the very best climates for growing hydrangeas are maritime, including Martha’s Vineyard and Nantucket. But we are also in a hydrangea sweet spot. If you walk the streets of Swarthmore and the Scott Arboretum from mid-June to mid-August, you will see an amazing array.
The quintessential hydrangea flower clusters, coveted by many, are either ball-shaped or flat-topped lacecaps, in pinks and blues. About 15 years ago, Martha Stewart featured cultivars of lacecap, Hydrangea macrophylla, on the cover of her magazine, sparking an international hydrangea renaissance.
Since then, there have been many new Hydrangea macrophylla cultivars, including the rebloomer Endless Summer. Bloomstruck is a new, more diminutive, version of Endless Summer. Some of my favorites include ‘Preziosa,’ which has reliably pink flowers in our soils, and Tiny Tuff Stuff, small in stature, with lacecaps of blue double flowers.
Native hydrangeas are loved for their summer flowers. ‘Annabelle’, a cultivar of the smooth hydrangea, Hydrangea arborescens, was introduced by J. C. McDaniel in 1962 and is often considered the best of the best. It blooms on current-season growth so can be pruned hard in March. In my yard, I prune them back to about 4 inches, so the shrubs are about 4 feet tall when they flower. In July, every stem is covered with a large, white, mophead flower. The flowers emerge lime green, turn pure white, and finally fade back to lime green and then tawny for the winter. In the wild, the smooth hydrangea grows in full shade, but it flowers best in part shade to full sun.
Invincibelle Spirit is considered a “pink Annabelle” — one of the holy grails of the hydrangea world. Invincibelle Limetta is a dwarf version of ‘Annabelle’ with massive flower heads.
The oakleaf hydrangea, Hydrangea quercifolia, is also native to the southeastern U.S. and is perfectly hardy. Starting in July, this shrub — which can reach 10 feet tall — proffers white cones of flowers atop leaves that look like red oak and turn purple-red in the fall. ‘Snow Queen’ is an older, but still popular, cultivar. ‘Snowflake’ has double flowers, which make the clusters so heavy they hang downward. ‘Amethyst’ has white flowers that quickly turn deep pink-purple. ‘Peewee’, ‘Syke’s Dwarf’, and ‘Munchkin’ are good for the small garden. ‘Ruby Slippers’ has white flowers that quickly turn pink and keeps to a nice 4-foot-tall dome. ‘Little Honey’ has butter-yellow leaves which perform best in part or dappled shade.
For me, Hydrangea paniculata is the quintessential flowering shrub. In July, the ‘Grandiflora’ (or “peegee”) hydrangea can reach small tree stature, with massive, erect, white panicles of lime green flowers. The flowers turn pure white and last into September. Because Hydrangea paniculata blooms on current-season growth, it can be left unpruned if there is room for a large shrub or small tree. Alternatively, it can be pruned hard in March for a smaller specimen.
In recent years, H. paniculata has gone through an amazing horticultural renaissance, with dozens of fantastic new selections introduced. One of my favorites is ‘Limelight’, with flowers that transition from lime green to alabaster white. If you want something smaller, Little Lime only reaches 4 feet tall with lime-green flowers, and ‘Dharuma’ has white flowers that fade to pink. It also stays small, making it perfect for the smaller suburban or urban garden.
I did not receive any questions this week, but I can offer you the answers to the two queries I got most often in my 25+ years at Scott Arboretum.
First, “When do I prune my hydrangea?”
You could probably write an entire book on this question. However, in general, don’t prune Hydrangea macrophylla. Only remove dead stems and old flowers. Selectively prune Hydrangea quercifolia. And, for H. arborescens and H. paniculata, which both bloom on “new wood” — stems newly grown that season — you can prune as hard as you want in March.
Second, “Why is my pink hydrangea blue?” Or vice versa. As I’ve said, the color of Hydrangea macrophylla flowers is influenced by the acidity, or pH, of the soil. The pH scale is from 1-14, with 7 being neutral. Generally, a pH of less than 7 makes blue flowers, and anything over 7 makes pink flowers. In Swarthmore, the pH of soils is generally below 7.
Send your gardening questions to editor@swarthmorean.com.
Andrew Bunting is vice president of public horticulture at the Pennsylvania Horticultural Society and vice president of the Swarthmore Horticultural Society.
